Structural adhesive bonding primers serve three basic purposes: (1) they protect the adherend surface from being affected by the workshop environment; (2) they inhibit corrosion of the bonded surface during its service life; and (3) they provide a compatible surface to which the adhesive can bond for long-term strength. They should have excellent mar resistance, protect the adherend from re-oxidizing, and be readily cleaned prior to bonding using standard workshop procedures. Additionally, they should protect the adherend during high humidity conditions; from salt corrosive environments; and withstand the effects of numerous fuels, hydraulic fluids, and lubricating oils. Standard industry tests include hundreds of hours at elevated temperatures or at 100% relative humidity or at exposure to salt fog (100% relative humidity and 95.degree. F. (35.degree. C.)) environment. Exposure tests for several days, both at ambient and elevated temperatures, to various fluids and chemicals are common modes of evaluation by industry. The primers should not adversely affect the performance of the bonding adhesive. Typical industry requirements are tensile shear strengths up to 41.4 Mpa (6,000 psi), peel strengths up to 11.34 N/mm (65 pounds per lineal inch), and long-term service (up to 6,000 hours) at elevated temperatures (up to 232.degree. C.). These prerequisites for a suitable adhesive bonding primer must be generally met before the primer achieves commercial acceptance.
The corrosion resistance required of a structural adhesive bonding primer is quite high, particularly when compared to a coating composition that is applied to a substrate for non-structural purposes. A structural adhesive bonding primer is part of a composite structure. It is the first layer applied to the metal adherend. The next layer is the adhesive. It is put on the adherend for the purpose of joining an adhering surface to the adherend. This results in the formation of a structural composite.
The criticality of stress corrosion of structural adhesive bonds is the subject of Bascom, Adhesives Age, pages 28, 29-35, April 1979. In this article, the author notes:
The most severe limitation to the use of structural adhesives is the susceptibility of the bond lines to attack by moisture. The effect of the moisture is generally considered a corrosion of the metal adherend, and evidence certainly exists to support this view. For example, in military operations in Southeast Asia, the seriousness of the problem was apparent from the extensive repair and refitting of aircraft caused by the delamination of aluminum skin and honeycomb structures. PA1 Presently, there is no clear understanding of the mechanisms involved in adhesive bond stress corrosion, nor are there any well established means of predicting bond durability under moist or wet environments. In fact, there is disagreement as to whether the primary attack is on the adhesive or the metal adherend. As for predicting bond lifetimes, there is no generally accepted test method for adhesive bond stress corrosion. PA1 "Their high VOC contents (80-90%) are a target that Regional Air Quality Management Boards, especially those in areas prone to substantial periods of air pollution, are aggressively seeking to regulate." PA1 Waterborne industrial coatings are attractive because they usually contain only small amounts of solvent and can meet the newer air pollution regulations. In addition, they minimize fire and health hazards. On the other hand, aqueous systems lack the versatility and frequently the quality of solvent systems. Because of sensitivity to atmospheric conditions, they often must be applied under stringent controlled conditions of suitable temperature and humidity. Problems of corrosion often necessitate the use of stainless steel equipment. Some problems can be met by careful choice of solvents used in most waterborne coatings. PA1 Waterborne coatings can be made by dispersing or emulsifying the resin binder by use of added surfactants. This technique leads to opaque liquids. Because some hard resins are difficult or impossible to disperse directly into water, the resin sometimes can be dissolved in a water-immiscible solvent, and the resulting solution dispersed by the use of added surfactants. In this case, the solvent aids subsequent film coalescence. Surface activity or water dispersability also can be introduced into resin molecules by chemical modification of the resin by functional polar groups such as the carboxyl group. PA1 Some very finely dispersed resins appear as clear as [sic] slightly hazy liquids; they frequently are described as soluble, solubilized, colloidal dispersions, micro-emulsions, hydrosols, etc. These resins contain built-in functional groups that confer water "solubility" upon the resin, and, normally, external added surfactants are not used. PA1 Waterborne resin binders can be classified as anionic, cationic, or nonionic. Anionic dispersions are characterized by negative charges on the resin or by negative charges on the surfactant associated with the resin. Cationic dispersions have a positive charge on the resin or on the surfactant associated with the resin. Nonionic dispersions are those that have been dispersed by addition of nonionic surfactants or that contain a built-in hydrophilic segment such as polyethylene oxide which is part of the main chain of a relatively hydrophobic resin molecule. PA1 (a) effectively bonds to an adherend for structural adhesive applications, PA1 (b) emits a VOC content of less than about 250 grams per liter, PA1 (c) is thermally stable, and PA1 (d) may be chromate-free or contain a minimal chromate content, yet possess excellent corrosion inhibition. PA1 2. The bismaleimide of formula V. showed a sharp melting point at about 160.degree. C. (320.degree. F.) and then reacted with itself to cure in the 200.degree.-300.degree. C. (392.degree.-572.degree. F.) range with a maximum rate at 253.degree. C. (487.4.degree. F.). PA1 3. The water soluble resin of formula IV. showed no self reaction exotherm before about 220.degree. C. (428.degree. F.) and there was a suggestion from the data that some thermal degradation occurred. PA1 4. That same resin modified with Cymel.RTM. 303 (see column 4, lines 55-56, of U.S. Pat. No. 4,800,215, supra), hexamethoxymethylmelamine resin with a 1.7 degree of polymerization, sold by American Cyanamid Company, Wayne, N.J. 07470, showed a reaction before 250.degree. C. (482.degree. C.), but indicated that the Cymel.RTM. 303 reaction with the hydroxyl groups in the resin occurred in the 250.degree.-320.degree. C. (482.degree.-608.degree. F.) range with a rate maximum at about 296.degree. C. (564.8.degree. F.). PA1 5. The water soluble resin of formula IV. containing both the bismaleimide of formula V. and Cymel.RTM. 303, showed a strong cure of the bismaleimide of formula V. at 130.degree.-232.degree. C. (266.degree.-449.6.degree. F.), with a maximum rate at 181.degree. C. (357.8.degree. F.). This was an activation of the bismaleimide reaction, dropping the rate maximum of bismaleimide from 252.degree. C., to 181.degree. C. The results suggest bismaleimide cure acceleration or an alternate cure reaction of the bismaleimide of formula V. with the water soluble resin such as by Michael addition or grafting. The Cymel.RTM. 303 cure with the resin hydroxyl groups still occurs over the 230.degree.-300.degree. C. (446.degree.-572.degree. F.) range, which is above practical cure conditions of 350.degree. F. (176.7.degree. C.) for the primer. PA1 6. The cure reaction of the water soluble resin with bismaleimide of formula V., without the presence of Cymel.RTM. 303, exhibited a strong reaction exotherm in the 140.degree.-230.degree. C. (284.degree.-446.degree. F.) range, with the maximum occurring at 161.degree. C. (321.8.degree. F.). This indicates that the cure does not involve the Cymel.RTM. 303 resin, and that the bismaleimide of formula V. reaction initiation at lower temperature still occurred. FNT .sup.5 See Clayton A. May, supra, pp. 1127-36.
The function of a structural adhesive bonding primer is to aid in keeping moisture from the adhesive-adherend interface and enhance the adhesion between the adherend and the adhesive. That action serves to minimize the impact of corrosion by acting as a barrier to moisture and passivating the adherend's surface from the impact of moisture that does penetrate to the adherend.
In the evolution of structural adhesive bonding primers, their formulations generally relied on dilute solvent solutions of modified epoxy or phenolic resins. These resins are generally considered innocuous, both being extensively used in food containers. However, materials used to cure these resins in adhesive bonding primers, such as amines, amides and imidazoles, may not be as innocuous. Solvents in the formulations have stimulated wide environmental concerns. The volatile organic compounds (VOCs) emitted by their evaporation from the adherend surface has been an ever increasing concern of industrial regulatory organizations.
Y. D. Ng and W. E. Rogers, in a paper entitled: "A Non-Chromated Water-Borne Adhesive Primer For Aerospace Applications" and given at the 33rd International SAMPE Symposium, during Mar. 7-10, 1988, discuss the environmental issues of adhesive primers. They point out that asbestos, at one time a favored raw material for adhesives, was virtually eliminated from the market since the early 1980's. They note that the aerospace industry has increased concern about using solvent-borne bonding primers.
Illustrative of this concern are the strict air quality mandated by the South Coast Air Quality Management District..sup.1 Though Y. D. Ng and W. E. Rogers indicate that most solvent-borne adhesive primers have little difficulty complying with 1987 SCAQMD Rule 1124 VOC limit for adhesive primers at 850 grams/liter, they fail to point out that typical epoxy/phenolic solvent-based adhesive bonding primers at about 10 percent solids emit VOCs into the atmosphere at levels approaching 800 grams/liter. Such VOC levels have been accepted because of the high performance the solvent based primers bring to the application. With ever increasing environmental concerns, such VOC levels are becoming unacceptable and there is a strong demand for epoxy based adhesive bonding primers that accommodate environmental concerns. SCAQMD has set the VOC limit for adhesive bonding primers at 250 grams/liter minus water, starting Jan. 1, 1993. This accords with the trend set for the coatings industry. FNT .sup.1 South Coast Air Quality Management District (SCAQMD) has jurisdiction over air quality in the Greater Los Angeles Basis in south California, U.S.A.
Such social reactions are stimulating the adhesive industry to find ways to reduce pollution by VOCs used as solvents in conventional adhesive bonding primers. Considerable emphasis exists to develop application technologies that reduce VOC emissions in adhesive bonding primer. A number of them have emerged to meet most but not all of the performance and application requirements, and at the same time meet emission requirements and regulations. One technology for overcoming the VOC problem involves the use of waterborne dispersions and solutions.
Clayton A, May, in his text entitled: EPOXY RESIN Chemistry and Technology, Second Edition, 1988, Published by Marcel Dekker, Inc., New York, N.Y., at page 766, makes the following characterization of waterborne coatings in general:
Waterborne coatings may be defined as coatings that contain water as the major volatile component and that utilize water to dilute the coating to application consistency. These coatings consist mainly of resinous binder, pigments, water, and organic solvent. The type of pigmentation and the method of incorporation of the pigment vary widely. It is usually easier to incorporate pigments directly into the organic phase where conventional dispersion techniques can be applied. . . .
The waterborne dispersions and solutions are to be contrasted with the water containing emulsion systems (oil in water varieties). In the latter case, the emulsion particles contain a concentration of highly volatile, water immiscible solvent plus a surfactant that keeps the emulsified particles suspended in the continuous water phase. Consequently, emulsions tend to have shorter shelf life stability than dispersions. During application, the emulsions rely on solvents to coalesce the deposited particles coupled with the surfactant, in order to form a continuous film that is free of pin holes and other defects. The surfactants (surface active agents) are normally higher boiling materials that are not easily removed from coatings deposited on the adherend. Such materials often remain as part of the coating and interfere with adhesion of a later applied adhesive. In addition, the retained surfactant remains a long term VOC problem. Thus, emulsions may not effectively address the VOC problem where there is the necessity to have a coated layer on the adherend that enhances the structural bonding of the adhesive and the adhering surface. The waterborne dispersions and solutions can effectively address the VOC problem as well as the structural bonding issues.
Waterborne structural adhesive bonding primers introduce entirely different selection of resin and cure system, and introduce formulation problems not dealt with in solvent based adhesive bonding primer systems. For example, waterborne adhesive bonding primers are not as resistant to corrosive environments as are the more conventional solvent-borne adhesive bonding primers. The conventional epoxy resins used in solvent-based systems are not water soluble or effectively water dispersible.
Epoxy based adhesive bonding primers typically contain inhibitors. The most effective, and hence, the most widely used inhibitors are chromate salts such as potassium chromate, barium chromate, strontium chromate, zinc chromate and the like. They are usually part of the pigment composition of the formulation. Chromate pigments are listed as toxic substances under SARA Title III, Section 313..sup.2 They are listed as chemicals known to cause cancer or reproductive toxicity under California Proposition 65..sup.3 Their removal from any formulation is desirable, so long as the formulation possesses satisfactory corrosion inhibition. FNT .sup.2 Superfund Amendments, a Re-Authorization Act of 1986 (SARA), Title III, Sections, 311, 312 and 313, United States Federal Regulation. FNT .sup.3 Proposition 65 (California Governor's list of "Chemicals known to cause cancer reproduction toxicity"), State of California (U.S.A.) Regulation.
Y. D. Ng and W. E. Rogers, supra, discuss the development of a waterborne structural adhesive bonding primer that uses the same multi-functional epoxy novolac resin as was used in "Hysol's EA 9205R (a 350.degree. F.) [176.7.degree. C.] service, solvent-bone adhesive primer." The less polar epoxy groups on the resin were transformed into more polar hydroxyl moieties. "Further treatment produced the cationic salt of the resin which provided the desired solubility and physical property characteristics." A combination of inhibitors are mentioned as replacements for chromates. They are stated to be proprietary. Properties of the proposed adhesive primer are discussed.
Clarke, U.S. Pat. Nos. 3,687,897, patented Aug. 29, 1972, and 3,789,053, patented Jan. 29, 1974, describe the reaction product of epoxides with isocyanates to produce oxazolidinones containing the unit structure: ##STR1## That work culminated in the development of coating compositions of dialkanolamine adducts of triglycidol ethers of trisphenols that is disclosed by Bertram et al., in U.S. Pat. No. 4,800,215, patented Jan. 24, 1989. The adducts of Bertram et al. may have the formula: ##STR2## In the above formula, m has a value equal to 3-o, n is equal to 2, and o is 1 or 2, preferably 1.
These water soluble coating resins are described as suitable for use with amine-aldehyde, urea-aldehyde or phenol-aldehyde curing systems. The patentees indicate that the resultant coatings have excellent thermal stability and/or elongation. There is no apparent indication that those coating resins are suitable for making structural adhesive bonding primers.
The water soluble resins of U.S. Pat. No. 4,800,215 may be obtained neat. They may contain water soluble ether solvents such as monomethyl ether of ethylene glycol, dimethyl ether of ethylene glycol, monoethyl ether of ethylene glycol, diethylene glycol, monomethyl ether of diethylene glycol, monomethyl ether of 1,2-propylene glycol, monomethyl ether of 1,3-propylene glycol, monoethyl ether of 1,2-propylene glycol, and the like.